1. Field of the Invention
The present invention relates to a ceramic heater used for an electronic-device manufacturing apparatus.
2. Description of the Related Art
In the process of manufacturing electronic devices such as semiconductor devices and liquid crystal devices, high-temperature processes such as chemical vapor deposition (CVD) and surface modification are employed. For example, in CVD, a semiconductor substrate to be processed is mounted on a ceramic heater provided in a reaction chamber of a CVD apparatus. Films such as a semiconductor film and an insulating film are formed on the substrate which is heated by the ceramic heater to a high temperature of approximately 500° C. or higher.
A ceramic heater is produced by bonding a cylindrical shaft to the lower surface of a flat plate made of aluminum nitride (AlN) (see Japanese Patent Application Laid-open No. 2003-142564). A heating element and a electrode for generating plasma are embedded in the plate. The upper surface of the plate serves as a mounting surface on which a substrate is mounted. The ceramic heater is fixed to the reaction chamber by the shaft.
In CVD, a corrosive gas is used as a process gas and a cleaning gas. In order to prevent the corrosive gas from being deposited on an outer edge portion of a substrate, there is a technique of supplying the outer edge portion of the substrate with a purge gas for removing the corrosive gas (see Japanese Patent Application Laid-open No. 2003-142564). Furthermore, in order to prevent a corrosive gas from going around the lower surface of a plate of a ceramic heater, there is also a technique of supplying a purge gas through an opening provided in the side surface of the plate, the purge gas for blocking the corrosive gas (see Japanese Patent No. 3976993).
For example, in plasma CVD, a corrosive gas containing fluorine is used as a process gas and a cleaning gas. In this case, a ceramic heater is exposed to fluorine plasma at a high temperature in a CVD or cleaning process. The fluorine plasma causes AlN of a plate to react with fluorine, and thereby aluminum fluoride (AlF3) is formed. AlF3 starts to sublimate at approximately 450° C. or higher, and accordingly the plate is corroded.
The thickness of the corroded plate is gradually decreased. Particularly, when the upper surface of the plate has a pocket shape, the following problem arises. Herein, a pocket shape refers to a shape formed of: a convex portion provided annularly at the outer edge portion of the upper surface of the plate; and a mounting surface which is surrounded by the convex portion, and on which a substrate is to be mounted.
When the upper surface of the plate has the pocket shape, the vicinity of the side wall of the convex portion provided at the outer edge portion becomes a gas reservoir where the corrosive gas stagnates. Moreover, of the mounting surface, a region formed between the side wall of the convex portion and the outer edge of the mounted substrate is not covered with the substrate, and is thus exposed to fluorine plasma during CVD. For this reason, of the mounting surface, AlN corrosion is significant in a region in the vicinity of the outer edge portion of the substrate, forming a dent in the region. The corroded dent changes the degree of contact between the substrate and the ceramic heater in the region; such a change in the degree of contact causes the temperature distribution in the substrate to be non-uniform. As a result, a problem arises that using the ceramic heater for a long period deteriorates the quality of a film to be formed on the substrate.
Such a problem has heretofore been dealt with the re-grinding of the upper surface of the plate having a dent formed therein. The dent formed due to the corrosion is approximately 10 μm to 100 μm in depth. Thus, the upper surface of the plate needs to be ground at a depth of approximately 100 μm in the re-grinding process.
However, the ceramic heater includes an embedded electrode for generating plasma approximately 1 mm below the upper surface of the plate. By grinding the upper surface of the plate, the thickness of a dielectric layer on the embedded electrode is decreased. This causes problems including: decrease in the resistance of the plate to thermal stress; change in the density of plasma generated in the reaction chamber; change in the temperature uniformity due to decrease in the heat capacity of the ceramic heater along with decrease in the thickness of the plate; and the like.
Meanwhile, sublimated AlF3 is deposited at a low-temperature portion in a form of fine particles. In the semiconductor fabricating process after the CVD process, the fine particles deposited on the back surface of the substrate are separated, which causes particle contamination.